Where is the surface emission for materials to create a light source from a plane?
Cycles:
Method 1:

Set the render engine to cycles
Go into the node editor
Check Use Nodes
Delete the Diffuse Node X

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Method 2:

Set the render engine to cycles
Go into the materials panel
Click Use Nodes
Change the Surface to Emission

 
Internal:

Check that you're using internal
Go into the materials panel
Set the emit value
Check shadeless (to look like cycles)